The invention pertains to a process for manufacturing polyester filament yarn for technical applications by spinning a polymer over 90% of the chains of which are composed of ethylene terephthalate units, with the spinning process having the following elements:
extruding the polymer in the molten state through a spinneret plate,
passing the thus formed filaments through a heated zone and a cooling zone in that order,
fixing the filament speed,
drawing the filaments to a length of 1.5 to 3.5 times their original length, and
winding the resulting filament yarn, with all elements being covered in a single process pass.
Such a process is well-known. For instance, European patent application EP 80 906 describes a process for the production of polyester filament yarn for technical applications by melt-spinning a polyester-containing polymer in which all process elements are covered in a single process pass. Such a process is also known as a one-step process. It is indicated in this publication that in such a process it is preferred to select a winding speed of less than 5500 m/min, since higher winding speeds will give rise to filamentation and difficulty in operation.
However, an increase in the winding speed is desired. When manufacturing polyester filament yarn for technical applications on an industrial scale, it is advantageous to produce the largest possible quantity of yarn per unit of time on an appropriate apparatus. One of the ways of increasing the quantity of yarn produced per unit of time lies in a higher winding speed.
U.S. patent specification No. 4,491,657 also discloses the process mentioned in the opening paragraph. This patent specification states that the winding speed of the yarn in such a one-step process is not less than 6.5 km/min. However, there are no examples in this patent specification of polyester filament yarns for technical applications made by such a one-step process at such a winding speed, nor is there any teaching on how to solve the problems which were found to occur when making polyester filament yarns for technical applications at such winding speeds.
Patent application WO 90/00638 indicates that in a one-step spinning process an increase in the spinning speed goes hand in hand with increased crystallinity of the as yet undrawn filaments. Yarn wound at a speed of about 4800 m/min can be obtained from undrawn filaments having a crystallinity in the range of 13 to 18%.